Pretreatment of enamel with polyvalent metal ions followed by APF application has already been shown to be more effective than APF alone in increasing enamel fluoride uptake and preventing caries-like lesion formation in vitro. By optimizing the conditions (pH, concentration, time) underwhich these treatments are made, it has been possible to reduce enamel surface loss. The cariostatic mechanism of the metal ions followed by APF treatment is not known. Recent work has suggested that two mechanisms may be involved; one, being the high fluoride levels on the surface and in the subsurface enamel affecting the chemical activities of calcium and phosphorus thus favoring remineralization; the other, depending on the presence of surface coatings, which may affect the diffusion of ions in and out of the lesion. These two possibilities are not mutually exclusive. This present work will investigate the part played by both of these mechanisms in cariostasis. This will be done by examining: 1) the distribution of various ions (metal, fluoride, calcium) in treated sound enamel and treated naturally occuring and artificial white spot lesions. 2) The morphological and histological appearance of treated sound enamel and treated naturally occuring white spot lesions. 3) The changes in ion distribution and morphological appearance will be investigated during lesion production in vitro. 4) The effect of the pretreatment regimen on the progression of existing natural and artificial white spot lesions will also be assessed. The analytical techniques for evaluating the distribution of ions will be microprobe analysis and fluoride by depth profiles observed by acid etch and microdissection methods. Surface and subsurface enamel will be examined morphologically by SEM and histologically by polarized light microscopy.